Re: Break Statements similar to C or C++

On 2007-05-31 07:55:47 -0400, grue@xxxxxxx (Timofei Shatrov) said:

Also there is loop-finish macro, which is often preferable to above-mentioned
methods.

Yes, but one should be aware that loop-finish is unlike break in that loop-finish causes execution to continue with the loop epilogue - the parts after finally or the automatic return of any accumulated values - while break in c and the methods shown above in lisp will cause control flow to exit the loop entirely - sbcl for example will delete the epilogue in such cases as unreachable code.


For example, this is probably not what one wants:

CL-USER> (loop named foo
for i = 0 then (random 100)
collect i ;;; you'll never see this accumulated list
when (> i 76) do (return-from foo i))
; in: LAMBDA NIL
; (RETURN-FROM FOO (SB-LOOP::LOOP-COLLECT-ANSWER #:LOOP-LIST-HEAD-1775))
; ; note: deleting unreachable code
; ; compilation unit finished
; printed 1 note
92


but this is:


CL-USER> (loop for i = 0 then (random 100)
collect i
when (> i 76) do (loop-finish))
(0 47 32 52 95)

and this could be too:

CL-USER> (loop named foo
for i = (random 100)
collect i
when (= i 91)
do (return-from foo 'exceptional-number-encountered)
when (< i 10)
do (loop-finish))
(65 54 37 50 38 93 12 99 94 83 95 77 39 62 82 51 62 88 2)
CL-USER> (loop named foo
for i = (random 100)
collect i
when (= i 91)
do (return-from foo 'exceptional-number-encountered)
when (< i 10)
do (loop-finish))
EXCEPTIONAL-NUMBER-ENCOUNTERED

loop-finish is more like a 'go' to a label placed at the start of the loop epilogue rather than a simple break, so one should use loop-finish when one *always* wants the loop epilogue to execute, and one should use one of the other techniques when one wants exceptional situations to terminate the loop a-la c's break - without executing any loop epilogue.

.

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